The present invention relates to novel intermediates in the synthesis of (xc2x1)-camptothecin and related compounds, and to a synthesis of camptothecin and related compounds via a 4+1 radical annulation.
As part of an antitumor screening program, Wall and coworkers identified the novel pyrrolo [3,4-b] quinoline alkaloid (S)-camptothecin in 1966. Wall, M. E., et al., J. Am. Chem. Soc., 88, 3888 (1966); Carte, B. K., et al., Tetrahedron, 46, 2747 (1990). The chemical formula of (S)-camptothecin is provided below. 
This compound had been isolated from the extracts of the camptotheca acuminata tree. In addition to its novel structure, camptothecin has two other unusual features: its quinoline nitrogen is not very basic, and its xcex1-hydroxy lactone is quite reactive. For a few years, camptothecin appeared to be an exciting lead compound for cancer chemotherapy. However, initial medical excitement waned because of the relative insolubility of camptothecin. Moreover, clinical trials of a water-soluble sodium salt derived by opening the lactone of camptothecin were abandoned because of unpredictable toxicity problems. The sodium salt is considerably less potent than camptothecin and its activity is now thought to result from lactonization to reform camptothecin in vivo.
Camptothecin was synthesized about ten times during the 1970s, although some later syntheses are modifications of earlier ones. Syntheses based on the Friedlander quinoline synthesis to construct ring B were most common. Ejima, A., et al., J. Chem. Soc., Perkin Trans. 1, 27 (1990); Earl, R. E. and Vollhardt, K. P. C., J. Org. Chem. 1984, 49, 4786; Ihara, M. et al., J. Org. Chem., 48, 3150 (1983); Cai, J. C. and Hutchinson, C. R., Chem. Heterocycl. Compd. 25, 753 (1983); Hutchinson, C. R., Tetrahedron 37, 1047 (1981); Cai, J. C. and Hutchinson, C. R., The Alkaloids: Chemistry and Pharmacology; Brossi, A. Ed.; Academic Press: New York, Vol. 21, p. 101 (1983); Schultz, A. G., Chem. Rev. 73, 385 (1973). Many syntheses are racemic, but resolutions have been reported. See Wani, M. C., et al. J. Med. Chem., 30, 2317 (1987). More recently, a chiral auxiliary approach to asymmetric ethylation was described. See Ejima, A., et al., Tetrahedron Lett., 30, 2639 (1989). Following the medicinal lead, synthetic interest in camptothecin peaked in the late 70s, and then began to wane.
Oncological and medicinal interest in camptothecin was reborn in the mid 80s when details about camptothecin""s unique mechanism of action began to unfold. Camptothecin acts on DNA through the intermediacy of the enzyme topoisomerase I. Hsiang, Y. H., et al., J. Biol. Chem. 260, 14873 (1985); Hsiang, Y. H. and Liu, L. F., Cancer Res., 48, 1722 (1988); Liu, L. F., Annu. Rev. Biochem., 58, 351 (1989); xe2x80x9cChemotherapy: Topoisomerases as Targets,xe2x80x9d Lance, 335, 82 (1990). The topoisomerases solve topological problems of DNA. Human topoisomerase I (100 kd) catalyzes the relaxation of supercoiled DNA by cleaving a single phosphodiester bond to form a temporary phosphoryl tyrosine diester. This intermediate is called the xe2x80x9ccleavable complex.xe2x80x9d The other end of the cleaved strand is free, and can xe2x80x9cunwindxe2x80x9d before the DNA chain is resealed by reverse of the original reaction. Topoisomerase I acts without cofactors, its reactions are fully reversible, and it is thought to be especially important for unwinding DNA (thermodynamically favorable) during replication. In contrast, topoisomerase II acts by cleaving the resealing (after strand passage) both strands of DNA, and its reactions are coupled with ATP hydrolysis.
There is now very strong evidence that camptothecin kills cells by binding to and stabilizing the covalent DNA-topoisomerase I complex in which one strand of DNA is broken (the cleavable complex). The progression from the ternary camptothecin/topoisomerase I/DNA complex to cell death is not well understood, and is the subject of intense investigation. Several lines of evidence (including the complete reversibility of ternary complex formation) indicate that the ternary complex does not simply tie up DNA, but itself actively initiates cell death. For this reason, camptothecin is often called a xe2x80x9ctopoisomerase poison.xe2x80x9d
Until very recently, camptothecin and its close relatives were the only known topoisomerase I poisons. In contrast, there are now many known antitumor agents that are topoisomerase II poisons. These include large classes of intercalators like the acridines and anthracyclines that were originally thought to interact only with DNA. Such topoisomerase II poisons may be inherently less selective than camptothecin because their interactions with DNA do not require topoisomerase II. Important non-intercalative topoisomerase II poisons include members of the podophyllotoxin class:
Camptothecin is being touted as an unusually important lead in cancer chemotherapy because of its selectivity. The (potential) selective toxicity of camptothecin towards cancer cells emanates from two sources: 1) camptothecin is highly selective for the DNA/topoisomerase I cleavable complex, and 2) replicating cancer cells contain elevated levels of topoisomerase I (1 5-fold increases over normal cells have recently been measured).
Recent tests in xenografts by Potmesil and coworkers were very promising. See Giovanella, B. C., et al., Science, 246, 1046 (1989). Racemic 9-aminocamptothecin was found to be very effective in treating mice carrying colon cancer xenografts. Indeed most of the mice in the study were cured by 9-aminocamptothecin at dose levels that were well tolerated. The improved efficacy of 9-aminocamptothecin compared to current drugs used in colon cancer chemotherapy (like 5-fluorouracil) was dramatic. 10,11-Methylenedioxycamptothecin also showed very good promise. Though it is still early, the significance of these results is very high. Human colon cancer is a major problem in clinical oncology, and one in twenty-five Americans will develop this disease during their lifetime.
Recent results are even more encouraging. See Giovanella, B. C., et al., Cancer Res., 51, 3052 (1991). It has been discovered that (S)-camptothecin itself can be formulated in 20% interlipid, and that this formulation is active both intramuscularly and orally. These treatments were far superior to the intravenous ones. With this formulation, non-toxic doses of camptothecin suppressed growth and induced regression of cancer in thirteen human xenograft lines including colon, lung, breast, stomach, ovary, and malignant melanoma. Camptothecin was much less toxic than its sodium salt, and was more effective than any other clinical drug tested.
Other close relatives of camptothecin are also emerging as excellent candidates for chemotherapy against a variety of tumor types. Several such compounds are undergoing clinical trials. Curran, D. P., xe2x80x9cThe Camptothecins: A Reborn Family of Antitumor Agents,xe2x80x9d J. of the Chinese Chem. Soc., 40, 1-6 (1993), the disclosure of which is incorporated herein by reference. See also Sawada, S., Chem. Pharm. Bull., 39,1446 (1991); Giovanella, B. C., et al., Science (Washington, D.C.), 246, 1046 (1989); Kingsbury, W. D., et al.; Med. Chem., 34, 98 (1991); Sawada, S., et al.; Chem. Pharm. Bull., 39,1446 (1991), Nicholas, A. W.; et al. J. Med. Chem. 33, 972 (1991).
The excitement about camptothecin recently increased to even greater levels upon the discovery that it is a potent antiretroviral agent. Preil and coworkers showed that camptothecin and relatives: 1) inhibited retroviral topoisomerase 1,2) prevented retroviral infections in healthy cells, 3) reduced and eliminated retroviral infections and infected cells, and 4) did not harm cells at useful dose levels. Priel, E., et al., AIDS Res. Hum. Retroviruses 7, 65 (1991). Topoisomerase II inhibitors were ineffective. These results suggest that camptothecin may represents a new avenue of investigation for the potential treatment of AIDS.
Given the current interest in camptothecins, new directions in the total synthesis of this family of compounds would be welcome.
Accordingly, the present invention provides a short, convergent total synthesis of (xc2x1)-camptothecin and related compounds using a novel 4+1 radical annulation followed by another cyclization to simultaneously assemble rings B and C of camptothecin and related compounds.
Generally, the present invention provides a method of synthesizing tetracyclic compounds having the general formula 
which are intermediates in many syntheses of (xc2x1)-camptothecin and related compounds. The conversion of these intermediates to (xc2x1)-camptothecin and related compounds is accomplished in two steps: hydroxymethylation and oxidation.
The synthesis of the tetracyclic intermediates comprises the step of a 4+1 radical annulation wherein the following novel precursor: 
is reacted with an aryl isocyanide such as phenyl isocyanide. Y is preferably selected from the group consisting of xe2x80x94N and xe2x80x94CR3. The aryl iscyanide may be unsubstituted, monosubstituted, disubstituted or trisubstituted.
R1, R2, R3 and R6are preferably selected from the following groups: hydrogen, normal and branched alkyl groups, haloalkyl groups, perfluoroalkyl groups, allyl groups, benzyl groups, propargyl groups, alkoxyl groups, halo groups, substituted amino groups, substituted acylamino groups, cyano groups, acyl groups, substituted hydroxy alkyl groups, substituted amino alkyl groups. R4 is preferably selected from primary or secondary alkyl, allyl, propargyl and benzyl groups. R5 is preferably selected from linear or branched alkyl groups or benzyl groups. Most preferably, R5 is selected from linear or branched alkyl groups in the range of C1 to C6.
The present synthetic route is useful for large-scale production of camptothecin and the production of new analogs of camptothecin for evaluation of biological activity.